Most of us realize, from experience, that many fruits can be picked and purchased in a non-ripe state, and then will continue to ripen. In fact, this characteristic is important to the storage and importing of fruits from around the world, allowing them to be be picked in a "green" state and so arrive at their destination ready to consume. Alternatively, the fruits can be purchased before they are ripe and the consumer can allow them to ripen at home. Knowledge of how the ripening process takes place has allowed the manipulation of the process, such as treating harvested fruits with ethylene gas, to accelerate the process when desired.

You may have purchased green bananas simply because you knew that ripe bananas would turn mushy before you got a chance to eat them all. Makes perfect sense and the banana was probably the first fruit that was harvested and then shipped in a green state, after it was discovered that they would ripen off the tree.

However, not all fruits will continue to ripen after they are harvested. Fruits that will, and fruits that won't can be separated, generally, into two categories: climacteric and non-climacteric fruits.

Climacteric fruits are fruits whose ripening process includes a large rise in respiratory state which is known as a climacteric rise. This is generally preceded by and elevated production of ethylene. Ethylene is a plant hormone that exerts a major influence on most aspects of plant growth and development, including tissue growth, abscission of fruits and leaves (separation from the plant), and ripening of fruit. You can read more about some of its effects in Do Apples Keep Potatoes From Sprouting?

After the climacteric rise, the production of ethylene gas drops off significantly. Climacteric fruits have a fast period of ripening during which they soften and develop flavor and aroma. These fruits generally have stored reserves of starch and during the climacteric rise these reserves are hydrolyzed by starch-degrading enzymes to simple sugars.

Unripe green and ripe yellow mangoes

Unripe green and ripe yellow mangoes

Whether the respiratory and ethylene increase is the absolute trigger of ripening in climacteric fruits, or whether it is a secondary consequence of some other process is still questioned. A number of fruits have been observed ripening on the vine without any increase in respiration. It could be that these phenomenon have to do with environmental conditions at the time of ripening.

Non-climacteric fruits do not exhibit these increases in respiration and ethylene production, but instead these process decline gradually during ripening. These fruits, therefore, do not have a rapid ripening phase but mature slowly and only while attached to the plant. As a general rule, non-climacteric fruits will not continue to ripen once they are picked and their eating quality will not change.

This does not mean, however, that certain favorable changes cannot take place after harvesting. For example, pineapples, a non-climacteric fruit, will soften after harvest. As well, even some non-climacteric fruit, pineapples and oranges included, can be artificially 'de-greened' by application of ethylene gas.

Below are lists of climacteric fruits that will ripen after harvest, and non-climacteric fruit that will not. Some fruits, such as guava, continue to be hard to pin down.

Non-climacteric Fruits Which Do Not Ripen After Picking

Bell Pepper

Blackberry

Cherry

Cucumber

Grape

Grapefruit

Lemon

Lime

Longan

Loquat

Lychee

Mandarin

Muskmelon

Olive

Orange

Pineapple

Pomegranate

Prickly Pear

Rambutan

Strawberry

Watermelon

How Do I Ripen Fruits More Quickly?

Assuming that the fruit the fruit is climacteric like those in the list above, you can use a very simple method to speed up the ripening process. In fact, this is a classic grade-school science experiment. Place the green or unripe fruit into a paper bag with a ripe fruit and fold down the top. It can be the same fruit but a ripe banana (not a mushy one) works very well. The ripening banana (or other fruit) is giving off a large amount of ethylene gas and when you enclose the fruits together inside a paper bag the ethylene becomes concentrated and this speeds up the ripening process of the unripe fruit. You can also use a breathable cotton cloth bag. But do not use plastic or any material that seals in moisture. It needs to be "breathable." Oxygen is part of the ripening process as well so you want some air to be able to pass through the material. Now, do not leave the fruit inside the bag too long or you'll get an over-ripe fruit! Go for 3 to 5 days.

Don't have a ripe banana? No problem, just put the unripe fruit(s) by itself into a paper bag. The fruit is going to begin to ripen anyway, and in so doing is going to give off ethylene. Putting it into the bag concentrates this ethylene so that the fruit is "breathing" it's own ethylene and so it will ripen quicker. Remember, same rule for oxygen as above.